1. Field of the Invention
The present invention relates to non-impact printing apparatus for printing characters, halftone images and the like with small picture elements (pixels) or dots.
2. Description of the Prior Art
In the prior art (as exemplified by U.S. Pat. No. 4,596,995) dot printers employing recording elements such as stylus, laser, ink jet, light emitting diodes (LED's), and the like are known. As an example, LED print apparatus comprise a multiplicity of individually addressable, point-like radiation sources that are arranged in a row or rows for exposing points upon a recording medium during movement of the photoreceptor relative to, and in a direction normal to, the row or rows. Driver circuits are provided for simultaneously energizing the radiation sources responsive to respective image or data bit input signals applied to the driver circuits during an information line period.
Usually, electrophotography produces positive prints when the original subject matter is in the positive form. However, for dot printers, it is often convenient to produce positive prints from negative exposure by so called "reversal" development. In reversal development, a recording medium is charged to a uniform unexposed primary voltage V.sub.0 and image-wise discharged to an exposure voltage V.sub.E. The exposed areas of the recording medium are toned at a development station having a bias voltage V.sub.B. The difference between V.sub.0 and V.sub.B is carefully maintained at a constant value to inhibit both background images and developer pickup. The difference between V.sub.0 and V.sub.E, herein referred to as .DELTA.V, is a factor in determing the image density.
Another factor in determining image density is the charge-to-mass ratio of the toner particles. For the same .DELTA.V, image density varies inversely with toner charge-to-mass ratio. This is of concern in multi-color machines since each color toner generally has a different charge to mass ratio and will therefore tone differently from other colors. If uncompensated for, this will cause a color imbalance in the final print.
One form of such compensation entails changing .DELTA.V between frames of different color-resolved latent images by adjusting the primary charge V.sub.0 in the interframe. For example, the recording medium is charged to a fixed value and then, according to the color-resolved latent image for the frame, the charge is reduced appropriately by means of a light source such as an electroluminescent panel. FIG. 1 is a graph of photoconductor voltage verses relative exposure for constant exposure and three values of V.sub.0 (i.e., 400 volts, 500 volts, and 600 volts).
At low levels of exposure (position "A" of FIG. 1), there is very little change in .DELTA.V as V.sub.0 changes between 400, 500, and 600 volts, and therefore inadequate compensation for changes in charge-to-mass ratio. At higher levels of exposure (position "B"), there would be adequate change in .DELTA.V, but the increased exposure requires more energy, and undesirably results in broader line widths.
Other disadvantages of the system desribed is the cost of the voltage-reducing light source (the electroluminescent panel) and its power supply. Further, since the bias voltage V.sub.B must follow V.sub.0, there must be as many V.sub.B power supplies as there are color development stations, or there must be a programable power supply, and the latter is as expensive as several constant-power supplies.